The present invention relates to a tungsten sintered compact target that is used upon forming, via the sputtering method, a gate electrode or a wiring material of an IC, LSI or the like, and to a method of producing such a tungsten sintered compact target.
In recent years, pursuant to the higher integration of very-large-scale integrated circuits (“VLSI”), studies are being conducted for using materials having lower electrical resistivity as the electrode material or the wiring material. Under the foregoing circumstances, high-purity tungsten having low resistivity and stable thermal and chemical characteristics is being used as the electrode material or the wiring material.
The foregoing electrode material or wiring material for VLSI is generally produced by way of the sputtering method or the CVD method, but the sputtering method is being widely used in comparison to the CVD method since the structure and operation of the equipment are relatively simple, deposition can be performed easily, and the process is of low cost.
While a tungsten target is demanded of high purity and high density, in recent years, as an electrode material or a wiring material for VLSI, a material with even lower electrical resistivity is being demanded in a film deposited by sputtering a tungsten target.
As described later, a tungsten sintered compact target is capable of attaining higher purity and high densification, and, while there are disclosures for achieving such higher purity and high densification, research and development regarding the abnormal grain growth of tungsten and deterioration in the target strength have not been conducted.
Conventionally, a tungsten sintered compact sputtering target was produced via pressure sintering using a graphite die. For example, this conventional method is described in Patent Document 1, Patent Document 2, and Patent Document 3. In the foregoing case, there is a possibility that C, as an impurity, will inevitably get mixed into tungsten. Moreover, while the type of die used is not particularly specified, Patent Document 4 and Patent Document 5 describe methods of achieving higher density.
The foregoing Patent Documents mainly aim to achieve the higher density of a tungsten target.
Otherwise, Patent Document 6 describes lowering the C content in a tungsten sintered compact target, and specifically discloses reducing the carbon content to be 50 ppm or less (the most reduced C content was 19 ppm in the Examples).
Moreover, Patent Document 7 discloses a technique of reducing the C content in a metal material (the most reduced C content was 10 ppm in the Examples) aiming to achieve the uniformity of the film and reduction of dust generation.
Moreover, Patent Document 8 discloses a technique of causing the C content to be 30 ppm or less (the most reduced C content was 6 ppm in the Examples) for producing a high-purity and high-density tungsten sintered compact target.
Meanwhile, it was discovered that there is a problem in that abnormal grain growth and deterioration in target strength occur during the production stage of a tungsten sintered compact sputtering target and consequently deteriorate the product yield. As a method of resolving this problem, the present Applicant discovered that the inclusion of phosphorus significantly affects the abnormal grain growth of tungsten and deterioration in target strength, and proposed causing the phosphorus contained in the tungsten to be 1 ppm or less as described in Patent Document 3.
Consequently, it became possible to prevent the abnormal grain growth of tungsten and improve the target product yield, and at that stage Patent Document 3 was an extremely effective method.
Nevertheless, while it was extremely effective to reduce the phosphorus contained in the tungsten, strictly speaking, abnormal grain growth still occurred, and additional improvement was required.
Generally speaking, a sintered compact is prepared via HIP in order to attain higher density and higher strength of the tungsten target, but since abnormal grain growth at this stage causes the problem of processing defects in the post-processing, there is a task of further reducing abnormal grain growth.
Other than the foregoing Patent Documents, Patent Document 9 describes adjusting the grain size and crystal structure via rotary forging, but the object of Patent Document 9 is not to prevent the abnormal grain growth of tungsten, and under the current circumstances there are no specific means for preventing abnormal grain growth.    Patent Document 1: JP 3086447 B    Patent Document 2: JP 2001-098364 A    Patent Document 3: WO 2009/147900    Patent Document 4: JP 2005-171389 A    Patent Document 5: JP 2007-314883 A    Patent Document 6: JP H05-093267 A    Patent Document 7: JP 2001-335923 A    Patent Document 8: JP H07-076771 A    Patent Document 9: JP 2012-180599 A